1. Field of the Invention
The present invention relates to a method of providing particulate materials with a protective coating. More particularly, the invention relates to the coating of particulate fertilizers such as ammonium nitrate with the reaction product of an amine and a siloxane.
2. Description of the Prior Art
In the past diatomaceous earth (DE) and treated clays have been used as a coating/conditioning agent for particulate fertilizers such as ammonium nitrate. However, for several reasons, diatomaceous earth is becoming increasingly unacceptable as a protective coating material for fertilizers. One reason is that the quality of diatomaceous earth, per se, has been declining as the better grades of the earth have been removed by mining. The inferior grades of DE have resulted in coatings which do not meet the stringent physical and chemical requirements for acceptable DE coatings. Storage and handling problems have resulted because of the inferior grades of DE which are being used.
Another disadvantage of using diatomaceous earth and clays is that there is always the undesirable problem of dust generation in the handling of diatomaceous earth since it is a relatively dry, small particle size solid. Usually, from about 1-1.5% by weight of DE is coated on ammonium nitrate particles. Not all of the DE, however, adheres to the fertilizer particles. This fact results in serious problems for ammonium nitrate manufacturing plants which are the likes of additional equipment maintenance, extra clean-up costs and the like. Moreover, there are serious customer handling problems involved with DE coated ammonium nitrate.
Yet another problem with diatomaceous earth is that the costs of agricultural grade diatomaceous earth have sharply increased over the past few years as the available supplies have decreased as a result of the diversion of supplies to other markets. Because diatomaceous earth suppliers no longer need the agricultural market as a consumer of their product, ammonium nitrate producers are placed in an increasingly more vulnerable and expensive position. A need, therefore, exists for a satisfactory replacement for diatomaceous earth as a fertilizer coating agent.
Various types of fertilizers have been coated with different materials in the past to produce a product of improved stability. Urea is usually coated with kaolin clay or is treated with formaldehyde to alleviate caking. However, kaolin, because of its very small particle size, is dusty. On the other hand, formaldehyde treatment of urea does not yield a completely satisfactory product.
Phosphate based fertilizers such as ammonium phosphate, superphosphate, homogeneous N-P-K fertilizers and the like, which when dried to very low water contents to reduce their caking tendencies, become quite dusty when handled. Fuel oil is frequently used as a surface coating material in order to suppress dust generation. While fuel oil exhibits some dust suppressing activity, it is not fully effective in this role, and is not a good use for an energy yielding substance.
Organic materials have been used in the past as coating materials for particulate fertilizers. Thus, Choi in U.S. Pat. No. 4,259,102 discloses a heterocyclic polymer which can be used as a fertilizer coating agent. Iwao, U.S. Pat. No. 3,660,070, shows the use of high molecular weight polymers of the vinyl and acrylate type as a coating agent. U.S. Pat. No. 3,388,990 provides a very similar type of disclosure. Passmore, U.S. Pat. No. 3,544,297, shows polyethylene, polyvinyl chloride and polyacrylonitrile as coating materials. Haruhiro, U.S. Pat. No. 3,475,154 discloses fertilizer particles coated with a multi-layered coating of a thermoplastic or thermosetting resin in order to provide a low water-soluble fertilizer. None of the above-discussed references provide any disclosure of either of the ingredients used in the coating agent of the present invention.
Hansen in U.S. Pat. No. 3,223,518 discloses a technique of providing a slow-release fertilizer by providing a particulate fertilizer substrate with a multi-layer coating, the outer layer being of a hydrophobic material. The reference further discloses the use of a primer coating which may be any substance selected from an extremely broad group of organic compounds. Hansen also describes that a major characteristic of the fertilizer product is its water durability which is characterized by solids which remain in the shell-like water-insoluble exterior coatings. Any fertilizer product which does not exhibit this characteristic, which includes the product of the present invention, presumably is outside the scope of the reference. Moreover, the fertilizer product of the present invention is not a slow-release multi-layer coated product having a hydrophobic exterior coating. Still further, the product of the reference possesses a large amount of coating material, i.e., about 7% by weight, while the amount of coating material applied to the present product is in most cases less than 0.1% by weight.
Bradley in U.S. Pat. No. 3,951,638, discloses a prilling process which at one point uses an organosilicon compound to prevent dust pick-up by fertilizer particles. Accordingly, the reference does not show the coating of fertilizer particles with an organic material. Assuming that the dust is an inert material, the organic material would seem to serve as a moisture barrier, which is a teaching contrary to the present invention.
Hijfte in U.S. Pat. No. 4,150,965 discloses the use of an alkylamine-mineral oil combination as a coating agent for ammonium nitrate fertilizer. The reference discloses a three-step coating process, while the present process is usually a one-step coating procedure. In the reference process an ammonium nitrate containing fertilizer is treated with the alkylamine while relatively hot, i.e., at least 5.degree. C. above the melting point of the amine or mixture containing the amine. In fact, the reference discloses that prills at 50.degree. C. cannot be coated satisfactorily, while in the present invention, on the other hand, superior coatings for ammonium nitrate can be achieved at temperatures less than 38.degree. C. Another disclosure in the reference which is contrary to the present invention is that unadulterated alkylamine must be added to the fertilizer in the first process step. That is, the amine cannot be premixed with another compound if acceptable results are to be obtained. In the present process, an amine is, in fact, premixed with another reagent, i.e., a silicone fluid, before application to the fertilizer particles. Another difference between the reference disclosure and the present invention is that the reference does not disclose the coating procedure to be effective on fertilizers which do not contain a diluent. The working examples of the reference disclose fertilizers which contain a diluent. A need, therefore, continues to exist for a technique of providing particulate fertilizers, particularly ammonium nitrate, with an improved coating material.